The publication WO 2006/108451 A1 originating with the applicant discloses for example a velocity-controlled load limiting device for a seat belt retractor comprising several masses to be oscillated which are located on the frame of the seat belt retractor. The masses are pivot-mounted on the retractor frame and, during the load-limited belt webbing extraction, with two teeth alternately engage into a toothed ring connected to the belt shaft. All told, the seat belt retractor is high-priced and its manufacture is time-consuming due to the oscillating masses located on the retractor frame. Furthermore, due to the oscillating masses, the seat belt retractor requires a relatively large installation space.
The German Patent Application 10 2008 049 931 A1 originating with the applicant as well discloses an improved load limiting device functioning on the basis of the same physical principle, in which two toothed parts are moved relative to each other in a load-limited manner, with one of the parts at the same time performing a wave-like feed motion during which the teeth alternately engage and disengage. The part performing the wave-like feed motion substitutes the oscillating masses, so that the load limiting device disclosed therein requires a significantly smaller installation space and shows a design that is much more simplified. In one embodiment, the part performing the wave-like feed motion is realized by a toothed disc which is connected to the belt shaft by a non-positive connection.
Furthermore, publication DE 10 2009 010 435 A1 discloses a seat belt retractor with a velocity-controlled load limiting device, in which the load limiting device for activation with one part is fixed at the frame of the seat belt retractor, and with another part can be coupled to the belt shaft. The part with which the load limiting device can be coupled to the belt shaft is designed as a rotatable toothed ring, into which a locking pawl mounted on the belt shaft engages. During the load-limited belt extraction movement, the torque is transmitted by the toothed ring via axial fingers to a toothed oscillating disc, which thereupon is driven to a rotational movement with a superimposed transverse oscillatory motion, and which thereby alternately engages into and disengages from teeth provided on the frame of the seat belt retractor. During this rotational movement, the oscillating disc is periodically accelerated and decelerated, and this is how the energy dissipation forming the basis of the load limitation is effected. One disadvantage of this embodiment is to be seen in that the toothed ring upon activation of the locking device needs to show a certain alignment relative to the engaging locking pawl in order that the locking pawl does not pseudo-lock at a front of the teeth of the toothed ring. The engaging movement of the locking pawl thus needs to be effected in synchronization with the alignment of the toothed ring, what in turn may be a problem insofar as the alignment of the toothed ring itself is defined by the same being mounted on the retractor frame. As the alignment of the toothed ring, due to manufacturing tolerances of the toothed ring itself and of the attachment points on the retractor frame, is subject to a certain unavoidable tolerance, the synchronization of the motion of the locking pawl is further complicated.
Furthermore, publication DE 10 2011 008 405 A1 discloses an improved velocity-con-trolled load limiting device for a seat belt retractor with a two-part belt shaft, in which one of the parts of the load limiting device is allocated to one part of the belt shaft, and the other part of the load limiting device is allocated to another part of the belt shaft. The load limiting device in this case is activated by one of the parts of the belt shaft together with the related first part of the load limiting device being locked in a vehicle-fixed manner in the event of an accident, and the respective other part of the belt shaft together with the respective other second part of the load limiting device starting to rotate relative to the first part of the load limiting device when a predetermined belt extraction force is exceeded.
Furthermore, modern seat belt retractors may be provided with pyrotechnical high-performance belt tensioners, which are activated in an early phase of the accident prior to the activation of the load limiting device and pull existing belt slack out of the safety belt, so that subsequently the occupant is coupled to the vehicle deceleration at an earlier stage.
The solution known from publication DE 10 2011 008 405 A1 describes a seat belt retractor with a tensioning device comprising a drive wheel mounted in a rotationally-fixed manner on a part of the belt shaft, which can be locked in a vehicle-fixed manner, and a driving apparatus in the form of mass bodies which can be accelerated pyrotechnically and can be engaged into the drive wheel. The velocity-controlled load limiting device here is located at an intersection point between the two parts of the belt shaft, adjacent to the tensioning device.
The problem resulting therefrom is that the tensioning device upon activation due to the close distance may have a detrimental effect on the load limiting device which is to be activated subsequently. Furthermore, package problems may arise therefrom, as the two technically complex assemblies are located on one and the same side of the belt shaft. Moreover, the velocity-controlled load limiting device in this solution is located between the legs of a U-shaped frame of the seat belt retractor, making the installation more difficult, in particular when the pre-assembled belt shaft shall be inserted into the frame from one side. The particular problem with it is that the openings in the frame cannot be dimensioned in any size.